I've run into the same problem. Good news: there's a workaround.
I, too, thought it was tied to having network users. But it's not.. at least not directly.
The problem is that if there aren't two or more local users, it hides the "Switch User" button (because apparently it thinks there's no other user you could possibly switch to). So the workaround is to create a couple of dummy local users. (Just go to the Users & Groups pane in System Preferences and add a few users.)
That will cause the "Switch User" button to appear. And yes, you can use it to switch to network users.